A system which executes a predetermined image processing based on a plurality of spectral image data obtained by capturing a subject illuminated with light having different optical properties at timings synchronizing with emission of the light is known. A concrete configuration of a system of this type is described, for example, in International Publication No. WO 2013/047054 A1 (hereafter, referred to as patent document 1).
In the system described in the patent document 1, white light emitted from a light source is sequentially filtered by a spectral filter to become light with narrow bands (each having a band width of approximately 5 nm) of 400, 405, 410, . . . , 800 nm. When a subject, such as a living tissue in a body cavity, is sequentially illuminated with the narrow band light filtered by the spectral filter, the subject is captured by an electronic scope at timings synchronizing with cycles of emission of the narrow band light. Then, using the spectral image data obtained by capturing the subject during emission of the narrow band light, the predetermined image processing is executed. For example, a color image substantially equal to a normal RGB color image is generated using three spectral image data whose center wavelengths are 435 nm, 545 nm and 700 nm, respectively, or a concentration index of oxyhemoglobin is calculated by using three spectral image data whose center wavelengths are 540 nm, 560 nm and 580 nm, respectively. The concentration index of oxyhemoglobin is used for, for example, generation of a color map image according to the concentration of oxyhemoglobin.
In the patent document 1, spectral images corresponding to respective narrow band light are obtained by capturing the subject at timings separated from each other on a time axis. It should be noted that there is no means to fix the electronic scope in the body cavity while the spectral images are captured. Therefore, even if an operator attempts to keep the electronic scope still during capturing of the spectral images, a relative position between the subject and the electronic scope in the body cavity (in other words, a position of the subject in a captured image) shifts. Accordingly, position shift occurs between the spectral images of the subject. For example, since positions of the subject shift with respect to each other between the spectral images of 435 nm, 545 nm and 700 nm, a color shift occurs in a color image of the subject generated by combining these spectral images. Furthermore, since positions of the subject in the spectral images of 540 nm, 560 nm and 580 nm shift with respect to each other, corresponding pixels (pixels having the same address) in the respective spectral images do not show the same portion of the subject. Therefore, an error may be contained in a calculation result of the concentration index of oxyhemoglobin.
In Japanese Patent Provisional Publication No. 2001-136540A (hereafter, referred to as patent document 2), a system which generates an RGB color image by capturing R (red), G (Green) and B (Blue) images through a frame sequential method is described. In the system described in the patent document 2, in order to suppress occurrence of color shift caused by position shift of a subject in a body cavity, a cross correlation value between the R, G and B images is calculated, a shift vector indicating a relative positional relationship between the images obtained when the cross correlation value becomes the maximum is detected, and the relative position between the images is corrected based on the detected shift vector.